1. Field of the Invention
This invention relates to electrical circuit over-current and/or over-temperature protection.
2. Background of the Invention
Positive temperature coefficient (PTC) devices are widely used in electrical circuit over-current or over-temperature protections. Such examples include U.S. Pat. No. 6,421,216, filed on Apr. 7, 2000, assigned to EWD, LLC and entitled “Resetable Overcurrent Protection Arrangement,” and U.S. Pat. No. 6,577,223, filed on Oct. 10, 2001, assigned to Uchiya Thermostat Co., Ltd. and entitled “Thermal Protector.” Both are herein incorporated by reference.
A conventional PTC circuit protection system 100 is shown in FIG. 1, connected between an electrical power supply 101 and an electrical load 103. The system 100 generally includes a bimetal switch 105 and a PTC element 107 thermally coupled and electrically connected in parallel. In a normal situation, the bimetal switch 105 is closed, and the operating current bypasses the PTC element 107 and is fed to the load 103 through the switch 105. In an over-current or an over-temperature situation, the bimetal switch 105 is opened. Current now flows through the PTC element 107 to the load 103 and heats up the PTC element 107. The heat generated by the PTC element 107 keeps the bimetal switch 105 in the open state. Furthermore, the resistance of the heated PTC element 107 is very large, and therefore the current through the load is limited to a very small value. Over-current or over-temperature protection is thereby achieved.
However, disadvantages exist with such conventional design in that the small current through the load 103, even in the over-current or over-temperature situation, may cause certain types of load to behave undesirably. For example, such small current may cause a compact fluorescent lamp (CFL) to flicker.